UNIVERSITY   OF   CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,    CALIFORNIA 

CIRCULAR  270 

September,  1923 

(Eevised  May,  1927) 

(Reprinted  October,  1929) 

A  FARM  SEPTIC  TANK 

W.  B.  HERMS  and  H.  L.  BELTON 


Forms  in  place  and  materials  ready  for  the  pouring  of  the  concrete. 


THE  FUNCTION  OF  THE  SEPTIC  TANK 

The  septic  tank  is  an  unventilated,  watertight  compartment  into 
which  all  raw  sewage  from  the  household  is  discharged  in  order  that 
the  more  solid  materials  may  be  liquified,  largely  deodorized,  and 
rendered  less  dangerous  preparatory  to  final  disposition.  It  is  not 
a  so-called  cesspool,  neither  is  it  a  panacea  for  all  sewage-disposal  ills. 
It  should  not  be  relied  upon  to  purify  the  sewage ;  that  is,  the  effluent 
from  a  septic  tank  is  not  much  less  dangerous  than  the  sewage  which 
enters  the  tank,  but  it  can  be  much  more  readily  disposed  of  and  that 
in  a  thoroughly  safe  manner. 


2  University  of  California — Experiment  Station 

The  useful  bacteria  which  cause  liquefaction  are  already  present 
in  the  sewage  and  require  only  the  proper  conditions — namely,  the 
exclusion  of  air  and  freedom  from  disturbance — to  carry  on  their 
good  work.  These  conditions  are  secured  in  a  dark,  unventilated  tank, 
provided  with  baffle  boards  to  prevent  agitation  and  to  allow  the 
entering  sewage  to  settle  quietly  to  the  bottom.  The  products  of 
liquefaction  are  to  a  large  extent  dissolved  in  the  water  of  the  sewage, 
forming  a  liquid  which  may  be  readily  disposed  of  and  leaving  behind 
a  small  portion  of  solid  organic  matter — comparatively  inoffensive — 
which  settles  as  a  sludge  and  forms  sooner  or  later  a  scum  on  the 

Re/nforcement 
I       e^sg.  rods     or  /1eavy_hog  fencing 


Inlet 


4" Single V 


Bottom 
Raffle 


Fig.  1. — Longitudinal  view  of  septic  tank. 

surface.  Cleaning  out  the  tank  is  usually  unnecessary  for  several 
years.  A  tight-fitting,  removable  cover  is  needed  to  allow  access  in 
case  of  trouble  or  possibly  necessary  cleaning. 

Typhoid  stools  and  other  infectious  matter  should  be  disinfected, 
preferably  by  boiling,  before  discharging  into  the  toilets.  Disinfectants 
must  not  be  added  in  quantities  to  the  sewage,  as  they  may  destroy  the 
useful  bacteria  in  the  tank. 


LOCATION 

The  septic  tank,  if  properly  constructed  of  concrete,  may  be  placed 
as  near  the  house  as  circumstances  permit,  even  beneath  the  porch  if 
necessary,  thus  securing  an  adequate  final  disposal  system  even  with 
very  limited  yard  space.  However,  it  must  be  kept  in  mind  that  a 
leaky  septic  tank  near  a  well  or  spring  is  dangerous.  The  tank  is 
usually  constructed  so  that  the  top  is  flush  with  the  ground  or  slightly 


Cir,  270] 


A  Farm  Septic  Tank 


lower.  This  will  place  the  top  of  the  outlet  line  twelve  to  sixteen 
inches  below  the  ground  level,  a  depth  which  has  been  found  to  be 
most  satisfactory  when  good  soil  conditions  for  drainage  exist.  A 
high  water  table  at  certain  seasons  of  the  year,  heavy  clay  loam,  or 
hardpan  near  the  surface  may  make  it  advisable  to  keep  the  top  of  the 
tank  six  or  eight  inches  above  ground  level  so  that  a  drainage  outlet 
nearer  the  surface  may  be  obtained. 


Fig.  2. — Half-section  view  of  septic  tank. 


TYPE  OF  TANK 


The  tendency  in  septic  tank  plans  is  in  the  direction  of  simplicity. 
Many  different  types  have  been  recommended  from  time  to  time, 
commonly  multiple  chambered.  However,  a  single-chambered  septic 
tank  provided  with  baffle  boards  to  control  local  currents  and  with 


University  of  California — Experiment  Station 


outlet  one  inch  lower  than  inlet  (figs.  1  and  2)  is  recommended.* 
Tanks  of  this  type  have  been  in  successful  operation  in  California  for 
over  ten  years. 


Fig.  3. 


-The  2-inch  by  10-inch  planks  laid  about  the  hole  are  later 
used  for  baffle  boards. 


SIZE  OF  TANK 

The  size  of  the  septic  tank  depends  upon  the  number  of  persons 
contributing  to  the  sewage.  For  a  household  of  ten  persons  or  less, 
the  inside  dimensions  of  the  tank  should  be  approximately  3  feet  wide, 
7  feet  long,  and  4  feet  deep.  A  smaller  tank  than  this,  even  for  smaller 
families,  is  not  recommended.  The  dimensions  for  larger  tanks,  as 
well  as  those  for  the  size  of  tank  described  in  this  circular,  are  given 
below : 


Inside  measurements 

Thickness  of  concrete 

of  people 

Width 

Length 

Depth 

Sides 

Ends 

Floor 

Top 

10 
16 
20 

3  ft. 
3^ft. 

4  ft. 

7  ft. 
9  ft. 
10  ft. 

4  ft. 
4  ft. 
4  ft. 

4  in. 

5  in. 

6  in. 

4  in. 

5  in. 

6  in. 

4  in. 
4  in. 
4  in. 

3  in. 
3^in. 

4  in. 

*  Grease  traps  are  not  necessary  when  waste  fats  and  grease  from  the  kitchen 
are  carefully  deposited  with  the  garbage. 


Cm.  270] 


A  Farm  Septic  Tank 


DIRECTIONS  FOR  BUILDING 

Preparing  for  Excavation. — Select  the  tank  location.  Take  two 
baffle  board  planks  and  place  them  3  feet  8  inches  apart,  Level  and 
hold  them  in  a  flat  position  against  stakes  driven  into  the  ground. 
Nail  these  planks  together  with  two  1  by  6-inch  boards  in  such  a  way 


Fig.  4. — View  of  form  for  septic  tank. 


as  to  form  a  rectangular  frame  with  inside  dimensions  7  feet  8  inches 
by  3  feet  8  inches  (fig.  3).  This  plank  frame  about  the  top  of  the 
hole  will  prevent  the  crumbling  away  of  the  earth  on  the  freshly 
poured  concrete  during  construction,  and  provides  a  working  edge 
from  which  a  plumb  bob  may  be  suspended  to  aid  in  digging  per- 


6 


University  of  California — Experiment  Station 


pendicular  walls.  Under  ordinary  conditions  the  hole  is  excavated 
to  a  depth  of  5  feet  below  the  bottom  of  the  frame.  This  places  the 
tank  5  inches,  and  the  top  of  the  outlet  pipe  16  inches,  below  the 


d 


Fig.  5. — Squaring  up  and  starting  the  side  wall  form. 


Fig.  6. — The  side  wall  sheathing  should  be  well  nailed  and  the 
boards  drawn  closely  together. 


surface.  Local  conditions,  however,  determine  the  depth  of  the  hole. 
If  the  earth  walls  can  not  be  used  as  an  outside  form,  the  hole  must 
be  made  larger  to  accommodate  forms  of  wood. 


Cir.  270] 


A  Farm  Septic  Tank 


The  Forms. — The  forms  should  be  made  of  common  lumber  sur- 
faced on  one  side,  the  outside  dimensions  to  correspond  to  the  inside 
dimensions  of  the  tank.  Figure  4  gives  the  necessary  dimensions  for 
construction  of  the  inside  form  for  a  tank  3  feet  wide,  7  feet  long,  and 
4  feet  deep,  capacity  for  10  persons. 

Side  Panel. — :The  sheathing,  which  may  be  1  inch  by  6  inches,  is 
nailed  firmly  to  the  2  by  4-inch  studding  (A,  fig.  4). 

End  Panels. — These  panels  are  held  to  the  2  by  3-inch  studding 
(B,  fig.  4),  which  fits  between  the  2  by  4-inch  studding  (A)  and  the 
end  of  each  side  wall  sheathing.     Two  number  8d  common  wire  nails 


Fig.  7. — Nail  the  1  by  2  baffle  board  cleats  firmly  in  position  after 
trimming  the  wall  sheathing. 


(0  and  D,  fig.  4),  at  each  corner  hold  the  2  by  3-inch  studding  (B) 
in  the  proper  position.  Six  1  by  6-inch  cross  braces  (E,  fig.  4),  three 
6  inches  below  the  top  and  three  6  inches  above  the  bottom,  hold  the 
forms  to  the  proper  width.  The  1  by  2-inch  strips  (F,  fig.  4)  nailed 
on  the  sides  of  the  form  to  make  a  groove  for  the  baffle  boards,  should 
be  slightly  beveled  on  the  edges  to  insure  easy  removal  of  the  side 
wall  forms. 

For  convenience  in  setting  the  form  in  position,  the  studdings  (A) 
should  project  2  feet  above  the  top  edge  of  the  wall  sheathing.  After 
the  form  has  been  lowered  into  the  hole,  it  should  be  blocked  up  level 
to  allow  for  a  4-inch  bottom  of  concrete,  and  squared  up  in  the  hole 
so  the  walls  will  be  of  the  same  thickness.     One  inch  by  six  inch 


8 


University  of  California — Experiment  Station 


hanging  boards  (fig.  11)  are  then  nailed  across  the  2  by  4-inch  stud- 
ding (A)  and  also  nailed  firmly  to  the  2  by  10-inch  plank  frame  which 
was  placed  about  the  top  edge  of  the  hole.  The  blocking  is  then 
removed  and  the  form  is  ready  for  concrete. 


Fig.  8. — Assembling  side  wall  forms  and  placing  the  1  by  6  cross  ties 
in  the  proper  position. 


Construction  of  Tank. — The  tank  should  be  constructed  with  4  to 
6-inch  sides  (see  table,  p.  4)  and  4-inch  bottom.  The  top  must  be 
3  inches  thick  and  reinforced.    A  1 :2y2  :4  mixture  of  concrete  material 


Cir.  270] 


A  Farm  Septic  Tank 


9 


is  recommended ;  i.e.,  1  sack  or  1  cubic  foot  of  cement,  2%  cubic  feet 
of  sand,  and  4  cubic  feet  of  crushed  rock.  Both  the  sand  and  the  rock 
should  be  clean  and  free  from  organic  matter.  The  sand  should  vary 
from  fine  to  %  inch  in  size,  the  rock  from  %  to  1  inch  in  size.  Clean, 
well-graded  gravel  may  be  substituted  for  the  rock  and  sand.  This 
should  be  mixed  in  a  proportion  of  one  bag  of  cement  to  4%  cubic 


orerx/ 

pane/s  is  X'minos  fyv/ce 
tne  w/ctth  of  corner  strip  '<J'/7g.4- 


Fig.  9. — Four  number  4d  box  nails,  one  at  each  corner,  hold  the  end  panels  to 
the  2  in.  x  3  in.  studding  B,  fig.  4. 


feet  of  gravel.  Enough  water  should  be  added  to  give  the  entire 
mixture  a  jelly-like  consistency.  Too  much  water  is  harmful,  as  the 
strength  of  the  concrete  is  reduced  by  adding  more  water  than  is 
necessary  for  a  workable  mixture.  The  concrete  should  be  thoroughly 
tamped  as  it  is  poured,  thus  insuring  smooth,  watertight  walls.  A 
1  inch  by  2  inch  V-shaped  strip  will  be  found  convenient  for  tamping 
and  working  the  concrete  into  place. 


10 


University  of  California — Experiment  Station 


The  concrete  should  be  poured  evenly  about  the  walls  until  about 
6  to  8  inches  has  been  placed.  (Do  not  crowd  the  forms  out  of  align- 
ment by  pouring  too  much  concrete  in  one  place.)  The  1  inch  by 
2  inch  by  5  foot  corner  strips  which  extend  12  inches  above  top  of  form 
(G,  fig.  4)  should  then  be  put  in  place  and  the  pouring  of  concrete 
continued  until  the  point  is  reached  where  the  6  by  6  by  4-inch  inlet 
and  outlet  boxes  (fig.  4)  should  be  placed.  A  small  nail  driven  into 
the  form  before  it  is  lowered  into  the  hole  will  help  in  locating  the 


Fig.  10. — The  completed  form  ready  to  be  lowered  into  the  hole. 


exact  position  of  these  boxes.  Continue  pouring  the  walls,  truing  the 
top  edge  with  a  trowel  or  block.  By  pouring  the  walls  first,  some 
concrete  will  work  out  from  under  the  wall  bottom  and  spread  over 
the  bottom  of  the  hole.  This  should  be  tamped  and  smoothed  into 
place  with  the  tank  floor,  which  should  be  poured  immediately  after 
the  walls.  Do  not  allow  the  concrete  floor  to  extend  any  higher  than 
the  bottom  of  the  forms  or  they  cannot  be  removed  readily. 

Construction  of  Cover. — Level  off  the  ground  nearby  and  stake  on 
edge  in  parallel  lines  3  feet  8  inches  apart,  two  pieces  of  1  inch  by 
4  inch,  8  feet  5  inches  long.  Cut  and  nail  in  position  1  by  4-inch 
cross  pieces  forming  four  rectangular  compartments  with  inside  dimen- 
sions 23  inches  wide  by  3  feet  8  inches  long  (fig.  12).  Fill  the  bottom 
of  these  spaces  with  fine  damp  earth  or  sand  to  a  smooth  level  surface, 
leaving  3  inches  for  concrete.     Pour  about  %  inch  of  concrete  and 


Cir,  270] 


A  Farm  Septic  Tank 


11 


immediately  place  reinforcing,  which  should  be  heavy  hog  fencing  or 
three  i/^-inch  steel  rods.  Continue  pouring  the  concrete,  leaving  the 
slabs  smooth  and  level  with  the  top  edge  of  the  form.  Cover  all 
freshly  poured  concrete  with  damp  earth  or  sacks. 


Fig.  11. — A  completed  tank  showing  the  1  inch  by  6  inch  hanging 
boards  supporting  the  form  in  proper  position. 


,  /Reinforcement 
"  e' '  ^-qaore  dors,  or 
heoi/c/  Ziog  fenc/ng 


Fig.  12. — The  cover  slab  forms. 


12  University  of  California — Experiment  Station 

Removing  Tank  Forms. — After  the  concrete  has  taken  its  initial 
set,  usually  in  about  one-half  hour,  the  1  by  2-inch  strip  (G,  fig.  4) 
at  the  four  corners  may  be  tapped  lightly  with  a  hammer  and  then 
drawn  out.  The  removal  of  these  strips  tends  to  prevent  the  cracking 
of  the  concrete  at  the  corners.  When  the  concrete  has  set  18  to  24 
hours,  the  forms  may  be  taken  out.  First  remove  the  1  by  6-inch 
hanging  boards  and  the  cross  braces  (E).  Remove  the  2  by  3-inch 
corner  stucldings  (B)  by  prying  them  inward  after  drawing  nails 
(C  and  D).     The  end  panels  may  then  easily  removed.     The  side 


rrrrrrnTnyrTTrrrrrrrrrrri  ?)/>r 


Fig.  13. — Burlap  or  tar  paper   (B)  keeps  the  loosely  filled  soil  from 
entering  the  line. 

wall  forms  can  be  removed  intact,  care  being  taken  that  the  concrete 
is  not  chipped  off  the  edges  of  the  grooves  into  which  the  ends  of  the 
baffle  boards  are  to  be  inserted.  Remove  the  6  by  6-inch  boxes  from 
the  wall. 

Connecting  the  Tank. — The  pipe  leading  from  the  house  to  the 
septic  tank  should  be  a  4-inch  vitrified  sewer  pipe  with  cemented  joints, 
laid  to  a  grade  of  %6  to  %  inch  per  foot  and  imbedded  firmly  in  the 
ground.  A  swab  made  from  a  piece  of  burlap  on  a  short  stick  should 
be  run  through  each  length  of  pipe  after  the  joint  is  cemented  to  clean 
out  any  cement  that  might  obstruct  the  inflowing  sewage.  The  inlet 
and  outlet  joints  should  be  firmly  imbedded  in  cement  mortar  (1  part 
cement,  2  parts  fine  sand).  The  outlet  should  be  placed  1  inch  below 
the  inlet.  The  baffle  boards,  preferably  of  redwood,  which  last  indefi- 
nitely, should  be  thoroughly  water  soaked  and  the  ends  trimmed  so 


Cm.  270] 


A  Farm  Septic  Tank 


13 


£Z     Tank 


Plan  A 


P/on  B 


DlZZlCZZlZpiZZHZZlL 


JL 
ji II iczzii II id 


Tank 


P/onC 


a 


31 — II      IL-JCZ3I — II IE 


zzicziczx-~ii — iizznr, 

J 

ji — it     ii irni — lizzie: 


D/vert/ng 
Box 


cjzzn 


■/3" » 


vm$ 


Plan 


It^1 n   ■  . '  ■■■■■■  ■■■    i 

Section 


Fig.  14. — Plan  A.     Single  line  used  when  a  minimum  amount  of  tile  is  necessary. 
Plan  B.     Two-branch  line  for  heavier  soils. 
Plan  C.     Three-branch  line  through  diverting  box,  used  where  greater 

area  is  needed  for  distribution  system. 
Lower  figure — Plan  and  section  of  diverting  box. 


14 


University  of  California — Experiment  Station 


that  they  fit  loosely  into  the  slots  in  the  tank  walls.  Concrete  cast 
iy2  inches  thick  by  the  method  described  for  the  casting  of  the  tank 
cover,  may  be  used  instead  of  redwood  baffle  boards.  The  cover  slabs 
should  now  be  put  in  place.  They  should  be  imbedded  in  mortar  with 
a  light  mortar  joint  between  the  slabs. 

FINAL  DISTBIBUTION  OF  EFFLUENT 

The  effluent  from  the  tank  may  be  utilized  for  the  sub-irrigation  of 
alfalfa,  berries  or  trees,  or  possibly  certain  vegetables  such  as  squashes 
and  beans,  but  not  for  any  vegetable  which  may  be  eaten  raw.     The 


Fig.  15. — A.  When  drainage  is  poor,  the  tile  should  be  laid  in 
twelve  to  eighteen  inches  of  loose  rock  or  gravel. 

effluent  is  ordinarily  discharged  through  4-inch  drain  tile.  The  first 
8  to  12  feet  should  be  vitrified  sewer  tile  and  laid  with  tightly  cemented 
joints  the  same  as  the  inlet  pipe.  The  remainder  of  the  drainage  line 
should  be  constructed  of  ordinary  4-inch  clay  drainage  tile.  It  has 
been  found  good  practice  to  lay  the  drain  tile  as  close  together  as 
possible,  covering  the  joints  on  top  with  a  strip  of  burlap  or  tar  paper 
(fig.  13)  to  keep  the  loosely  filled  earth  from  entering  the  line. 

The  arrangement  of  the  disposal  tile  (fig.  14)  after  the  first  8  to 
12  feet  are  laid,  may  be  in  the  form  of  a  two  or  three-tine  pitchfork 
in  order  to  conserve  space  and  to  assist,  if  desirable,  in  sub-surface 
irrigation.    A  diverting  box  (fig.  14)  is  recommended  when  three  or 


Cm.  270]  A  Farm  Septic  Tank  15 

more  branch  lines  are  used.  A  fall  of  1  inch  to  10  feet  is  commonly 
used  in  sandy  soil.  One  inch  in  60  feet  is  recommended  for  heavier 
soils,  with  a  slightly  greater  opening  in  the  tile  joints.  When  drainage 
conditions  are  very  poor,  or  if  for  other  reasons  the  tile  must  be  laid 
close  to  the  surface,  it  is  recommended  that  the  drain  tile  be  laid  in 
12  to  18  inches  of  rock  or  gravel  in  a  ditch  about  12  inches  wide 
(fig.  15).  When  the  drain  is  laid  on  soil,  it  should  be  firmly  packed, 
for  the  effluent  seeping  from  the  open  joints  may  cause  loosely  packed 
soil  to  settle,  thus  causing  a  settling  of  the  drain  tile  and  perhaps  the 
clogging  up  of  the  system.  The  distributing  system  should  be  located 
as  far  away  from  the  source  of  drinking  water  as  possible  to  prevent 
contamination  by  seepage.  Under  no  conditions  should  the  effluent 
be  drained  direct  from  the  tank  to  a  dug  well  or  cesspool,  if  there  is 
any  possibility  of  the  seepage  from  this  cesspool  contaminating  the 
domestic  water  supply.  Sixty  feet  is  the  minimum  amount  of  drain 
tile  to  be  used  for  a  distributing  system  under  good  soil  conditions. 
More  tile  should  be  added  if  needed.  One  hundred  feet  is  the  maximum 
for  any  one  branch  line  of  a  system. 

After  all  cemented  joints  have  set  firmly,  run  150  or  200  gallons 
of  water  into  the  tank  and  it  is  ready  for  use. 


MATERIALS  REQUIRED 

The  materials  necessary  for  a  septic  tank  3  feet  wide,  4  feet  deep, 
and  7  feet  long,  suitable  for  ten  persons,  are  as  follows : 

Concrete  Materials. 

1%  cubic  yards  crushed  rock  %  inch  to  1  inch  in  size. 

1  cubic  yard  sand. 

12  bags  cement. 

(If  crushed  rock  is  not  available,  2  cubic  yards  of  clean,  well 
graded  gravel  may  be  substituted  for  rock  and  sand.) 
Tile. 

Two  vitrified  single  branch  Y  sewer  tile,  size  4  inch. 

*Vitrified  bell  neck  sewer  tile,  size  4  inch. 

*  Standard  drainage  tile,  size  4  inch. 

Reinforcing. 

4  pieces  of  heavy  hog  fencing,  size  23  X  43  inches,  or 
12  pieces  %  inch  steel  rods,  3  feet  7  inches  long. 


Amounts  not  figured  as  local  conditions  govern  quantity  required. 


16 


University  of  California — Experiment  Station 


Baffle  Boards. 

3  pes.  2"  X  10" 


10'  Rgh.  Com.  Rdw.  or  Cedar. 


All  common  lumber  surfaced 
one  side. 


Cover  Forms. 

3  pes.  1"  X  4"  X  12' 

Form  Lumber. 
Side  Walls 

9  pes.  1"  X  6"  X  14' 
End  Walls  and  Braces 

6  pes.  1"  X  6"  X  12' 

2  pes.  1"  X  4"  X  8' 
Hanging  Boards  and  Boxes 

1  pc.  1"  X  6"  X  12' 
Baffle  Cleats  and  Corner  Strips 

3  pes.  1"  X  2"  X  14' 
Long  and  Center  Studding 

2  pes.  2"  X  4"  X  16' 
Corner  Studding 

1  pc.  2"  X  3"  X  16' 

Nails 

3  lbs.  No.  8d  common  wire  nails 
2  lbs.  No.  6d  box  nails 

COSTS 

The  average  cost  of  a  septic  tank  is  $27  to  $36  exclusive  of  labor. 
The  labor  of  two  men  for  two  days  is  necessary  to  construct  a  tank 
under  average  conditions. 


ACKNOWLEDGMENT 

The  authors  wish  to  acknowledge  indebtedness  to  E.  J.  Stirniman 
for  his  valuable  assistance  in  the  preparation  of  this  circular  and  to 
J.  P.  Fairbank  for  his  suggestions  for  the  improvement  of  this  reprint 
of  the  original  circular. 


10m-10,'29 


